Magnesium hydroxide suspensions with a relatively low concentration of magnesium hydroxide have been traditionally used as a laxative and an antacid and are commonly known as "milk of magnesia." The current USP edition defines "milk of magnesia" as a suspension of magnesium hydroxide, each 100 g of which contains not less than 7.0 g and not more than 8.5 g of Mg(OH).sub.2. It may contain 0.1 percent of citric acid, and may contain not more than 0.05 percent of a volatile oil or a blend of volatile oils, suitable for flavoring purposes.
The "milk of magnesia" preparations now on the market and made by diluting a magnesium hydroxide paste with water suffer from several major drawbacks in spite of being described as "stable suspensions." These preparations invariably separate into two (2) distinct layers with the clear aqueous layer forming the upper layer. The lower opaque layer is not uniform and contains increasing densities of the magnesium hydroxide as one goes from the top to the bottom. If the consumer fails to shake the bottle vigorously prior to use, he fails to obtain the desired dosage. If the marketed preparation of such a product is not used up in a relatively short time, then the partially used preparation presents even greater problems as the magnesium hydroxide paste tends to dry between the cap and the neck of the bottle and there ensues a slow and steady evaporation of water from the preparation, which sometimes results in a solid mass instead of the liquid suspension in the bottle. These separated preparations are difficult to reconstitute to a uniform constitution by gentle shaking, and frequently even prolonged vigorous shaking of such preparations will not result in a uniformly reconstituted product. The consumer tends to get a lower than intended dose upon initial use of the preparation and a higher than intended dose as the product is used up. Furthermore, the milk of magnesia products now on the market have poor "freeze-thaw" stability. These products also have an alkaline, chalky taste which prevents wide acceptance by the consumer.
The proposed FDA monograph on laxatives recommends a dosage of about 30 to 60 ml of milk of magnesia, i.e., the milk of magnesia preparations now on the market and containing from about 7.0 to 8.5 percent by weight of magnesium hydroxide. This amounts to taking about 6 to 12 teaspoons of the product at one time, which is inconvenient and unacceptable to some consumers. While more concentrated magnesium hydroxide preparations would solve this problem, such preparations having good stability and other desirable properties are not available.
It is accordingly a main object of the present invention to provide a process for the preparation of such pharmaceutically elegant concentrated magnesium hydroxide preparations.
It is another object of the present invention to provide a concentrated aqueous magnesium hydroxide preparation which does not readily settle or cake on standing.
It is still another object of the present invention to provide a concentrated aqueous magnesium hydroxide preparation which has freeze-thaw stability and is palatable.
It is a further object of the present invention to provide aqueous magnesium hydroxide preparations having the above-mentioned properties and containing from about 16 to 24 percent by weight/volume of magnesium hydroxide.